Addendum 



A New Method for the Determination of the Quantum 



Requirement 



Recently Warburg and Krippahl* worked out an improved compensation 

 method for determining the quantum requirement of Chlorella. In using the 

 compensation method, dense cell suspensions are used so that the resulting 

 great respiration has to be compensated with diffuse bluish light (20% blue 

 and blue-green, 80% green, yellow and red). The compensation light is 

 provided by a 100 watt metal filament lamp with a reflector fixed above the 

 thermostat. Light absorption is nearly complete. As it is not possible to 

 measure the absorbed intensity of such a diffuse compensation light with a 

 bolometer, the manometer vessels are replaced by actinometer vessels con- 

 taining a solution of pheophorbide and thiourea in pyridine (see § 24). The 

 uptake of O2 in the actinometer divided by the O2 evolution in the cell suspen- 

 sion equals the quantum requirement for the compensation light. The de- 

 termination of the quantum requirement for the measured light is carried out 

 in the usual way with the two-vessel method, the light beams entering the 

 vessels from below. Respiration is compensated ; the intensities are measured 

 with the bolometer and the degree of absorption which varies between 93 and 

 95% is found with the aid of the Ulbricht sphere. 



It has been found that the quantum requirement decreases with decreasing 

 light intensity. At i = 0, a limit of \/^p is obtained which has the value 3.06 

 for the compensation light and 2.85 for the measured light (Fig. 71). The 

 low value of 2.85 is very near to the theoretical quantum requirement 2.7 

 at 100% efficiency (see § 26). It is of course irrelevant that the value for the 

 quantum requirement is not a whole number. According to the one- 

 quantum reaction, one quantum causes the splitting of one molecule photolyte 

 producing one molecule O2. As discussed in § 35, at the quantum require- 

 ment 3 of the over-all reaction, 2/3 of the O2 evolved are used to produce the 

 energy necessary for renewal of the photolyte. If the Oo, reacting back were 

 somewhat less than 2/3, the quantum requirement of the over-all reaction 

 would of course be somewhat less than 3. 



It may seem at first glance that quantum yield determinations in photo- 

 synthesis should be as easy as those in simple photochemical reactions. 

 However, this is not the case and it must be repeated once again that the cells 

 must be forced to utilize the energy of light at its maximum. As a matter of 

 fact they do not do so when they are not cultivated under fluctuating illumi- 

 nation, when they are illuminated without blue-green light, when the neces- 



Warburg, O. and Krippahl, G. Zsclir. Naturf., 15b: 190, 1960. 



187 



